It has long been recognized that higher areal density recording can be achieved with a high degree of reproduction if servo signals, i.e., track location indicating signals, could be superposed with data signals in a magnetic storage system. Although recognized, a technique for accomplishing this goal has never been commercially exploited because of the degradation of the "buried servo" or track-position indicating signals, the interference with the data signal by the buried servo, and the expense of providing a multiple layer magnetic storage system, one layer for the position indicating signals and another for the data signals.
Buried servos have been long recognized as an effective means for faithful track positioning control. The prior art systems shown and described in U.S. Pat. Nos. 4,313,140, 4,314,289 and 4,318,141 employ a single-gap transducer for recording a data signal while at the same time sensing prerecorded servo control signals. To effect this simultaneous writing and reading operation, it was necessary to minimize the effect of the counter electromotive force (abbreviated counter emf) induced by the recording data signal on the signal-to-noise ratio of the servo control signals. For this purpose, the recording circuit of the system included an AC bias and a high-pass filter to improve the signal-to-noise ratio of the servo control signals. A notch filter was further required to suppress frequencies of the data signal to reduce the counter emf.
However, the signal-to-noise ratio of the servo control signals was found to be still not satisfactory and the use of an AC bias and filters added to complexity with an attendant cost increase.
Notwithstanding these problems, the art has attempted to obtain higher areal density recording with a high degree of faithful reproduction. An early teaching of superposed data and servo signals is found in the F.J. Sordello U.S. Pat. No. 3,404,392 which teaches using a dual-layer magnetic coating on a disk substrate. Servo signals are first recorded by being placed in a lower layer of the substrate for indicating track positions. Data signals are recorded in a higher layer subsequent to the recording of the servo signals. Both the servo and data signals are then sensed. For each data recording operation, an erase gap precedes a read/write gap so that the data signals can be successfully recorded in the upper layer. During readback, the data signals are read back by a single gap, and two sets of servo signals are simultaneously read back. The servo signals are linearly recorded sine waves written in concentric circular tracks and they exhibit a low frequency. Adjacent servo tracks are written at different frequencies. Each data track is defined as being superposed over one-half of an odd numbered servo track and one-half of an even-numbered servo track. The data signals and servo signals exist in the magnetic medium independently of the other. In this dual-layered record medium, recording the data signals at a high frequency, at least three or four times the frequency of the servo signals in the lower layer, enables the data signals to be separated from the servo signals. Sordello achieves this independence requirement by providing a record disk having a lower level with a higher coercivity than the upper level which stores the data signals. This arrangement allows the lower level servo signals to remain undisturbed by subsequent writing and rewriting of data signals in the upper layer. The single gap permits perfect alignment for simultaneously reading the control or servo signals with data signals.
M.R. Cannon in U.S. Pat. No. 3,765,005 discloses using a clock-bias signal at the upper end of a data signal frequency spectrum. Also, U.S. Pat. No. 3,821,798 shows a low-frequency control signal recorded at a lower end portion of the data signal pass band.
A later reference showing "buried servo" is Beecroft et al., U.S. Pat. No. 3,956,769. Beecroft et al. teach the use of two separate gaps in a transducer, one for servo signals and the other for data signals. U.S. Pat. No. 4,313,140 to Keidl shows read and write circuits connected to a magnetic medium. A control signal circuit having band or low-pass frequency characteristics is connected to a transducer for receiving sensed control signals simultaneously while the write means is supplying information-bearing signals for recording. The recording and sensing operations occur simultaneously via a single-gap transducer. The recorder also has an information-bearing signal readback circuit.
M.K. Haynes in U.S. Pat. No. 4,318,141 discloses a magnetic recorder having a single gap magnetic transducer adapted to exchange signals within an adjacent magnetic record medium which has a hard recorded control signal, i.e., the signal exhibits a retentivity characteristic exemplified by signals recorded using a head having an effective gap length greater than a gap length to be used with subsequent data signal recordings. The control signal is preferably a narrow band, low frequency sinusoidal-type signal. The single-gap transducer is connected to a recording means having high-pass frequency characteristics for supplying information-bearing signals to the transducer for recording on an adjacent magnetic surface. Also connected to the transducer is a control signal circuit having low-pass frequency characteristics for receiving sensed control signals simultaneously while the write means is supplying information-bearing signals for recording. The operation is enhanced by an AC bias means connected to a transducer for supplying an AC bias signal simultaneously with said recording and sensing operations. The recorder also has an information-bearing readback circuit.
The above prior art in general essentially left a spectral "notch" around zero frequency range (DC) for "buried servo." The "notch" was insufficient. It was found in some cases that the servo signals attenuated gradually by the writing of data into the disk until reaching some asymptotic power around 6 dB lower than the original signal power. The servo signals often interfered with the data signals at readback. In the write process, some of the buried servo information was erased.